Electrolytic polishing of metals



July 25, 1950 E. DER MATEOS'IAN ELECTROLYTIC POLISHING 0F METALS FiledJune 20, 1945 FREEZING MIXTURE SIZE-E FREEZING MIXTURE EDWARD DERMATEOSIAN Patented July 25, 1950 UNITED STATES PATENT OFFICE 7 Claims.

(Granted under the act of March 3, 1883, as amended April .30, 1928,;370 .0. .G. 1757) My invention relates to a method and anwelectrolyticbath for the .anodic or electrolytic polishing of metals.

Electrolytic polishing methods usedlin the finishing of metal surfaceshave remained highly empirical practices in which specific electrolyticsolutions having polishing properties only for certain metals areeffective. In general, the art shows that the solution used as thepolishing electrolyte is strongly acidic and .quite specific for themetal being polished in order to obtain the result usually described.Accordingly, it is an object of my invention to provide a compositionfor an electrolyte and a method of polishing which do not have suchspecificity and can be used to polish many metals with only minoradjustments in the composition of the electrolyte.

Polishing solutions frequently contain heavy concentrations of strongoxidizing agents, such as chlorates, and conditions in the electrolytenot infrequently develop whichresult in the occurrence of explosivereactions within the electrolyte. Hence, it is another object of myinvention to provide an electrolyticpolishing solution and method whichcan be carriedout-conveniently and in a manner such that the solution atall times will be free of high concentrations of oxidizing agents and,consequently, the danger of the occurrence of an explosion iseliminated.

It is another object of my invention'to provide a method of electrolyticpolishing whereby a solution of electrolyte prepared according to'theprinciples of myinvention can be causedto -polish many metals.

It is a further object of my invention toprovide an electrolyte andmethod of polishing and treating metals and alloys to produce 'finishedsurfaces of a degree .of polish sufiicient for reproduction forexamination under the electron microscope.

It is another object of my invention to provide a method of polishingalloys having "more than one phase present whereby one of the phases ofthe alloy can be selectively polished so that'the surface of the alloycan beexamined for the presence of that phase, the other phasesremaining unpolished or etched.

Other objects and advantages of my invention will in part be obvious andin part appear hereinafter in the .following detailed description of itsprinciples and several of its embodiments.

My invention, accordingly, "consists .of..;ele.c-

trolytic bath and the method of polishing-metals including therelationship of .one onmoreaofrthe constituents of the bath to each.of1the .others thereof and the relationship of one or more of '.cordingto my invention.

Inorder to polish, an electrolytic bath should cause polarization of theanode and it should have the capacity for carrying a heavy current, fora necessary-condition for thelproduction of polarization within the cellis the employment of a highcurrent density. The bath should also causethe anode metal, or the metal being polished, to go into solution and,at the same time, liberate oxygen at the anode. It should also produceconcentration polarization within the cell.

Ihave discovered a composition for an electrolyte which permits thecontrol of the abovenamed eifects and includes simply a solution of .asubstantially .neutral salt, such as an alkali metal salt of ,a mineralacid in water and an organic .solvent such asalcohol. The concentrationand pH of the solutionare determine'd'by the polishing problem onlhand.The pH of the solution is adjusted until polishing takes place at theanode, which value of pH 'I call the polishing pH of the solution.

'It is necessary that'thesalt used have a solubility high enough in thewater-organic solvent mixture to carry asufficiently heavy current tocause polarization of the anode. In general salts such as alkali metalsalts of the mineral acids, .1. e., sodium, potassium and lithium saltsof nitric, hydrochloric, hydrobromic, 'hydroiodic, hy-

thus permitting the applicationof current densities to specimenssufficient to accomplish polishing. It is' a -matter of common knowledgethat some salts of the mineral acids particularly the phosphates are notneutral in solution.

However, .in such cases, .solutions of the salts scan .belmadelup .withisuilicientgfree acid to: :counteract the inherentbufifer nature of thesalt-:thus

providing a substantially-neutral salt solution, as a starting point.

The organic solvent can ,be any completely water-miscible organic liquidor solvent such as ethyl alcohol, methyl-alcohol, glycerine, ethyleneglycol, acetone, isopropyl alcohol, Cellosolves, water-miscible estersand like organic compounds. Apparently the function of the organicmaterial in the ele rolyte is to reduce the ion velocity and therby toaid in the establishment of concentration polarization at the cellanode.

The pH f the solution of the salt in the waterorganic so vent mixture isadjusted by the addition of a small amount of astrong base or acid toadjust it to the polishing pH. Sodium hydroxide can be used as thealkali and, for acidification of the solution, such acids as, acetic,nitric, oxalic, citric, sulfuric, or hydrochloric acids can be used. Ingeneral, it is preferable, although not essential, that the acid used toacidify the solution be one that will supply the same acid radical asthat in the solution as part of the salt. This is particularly tobeobserved where there is possibility that ingredients of theelectrolyte will react with each other.

When the electrolytic baths are freshly prepared, it is not immediatelycertain that they will polish the metal it is desired to polish. Simpleexperimental observation will tell the operator after a few seconds ofelectrolysis of his specimen whether or not the polishing is takingplace for,

doing. In the drawing, represents a vessel for holding electrolyte 2|.Battery 22 supplies current to the cell by way of the cathode 23 andanode 24, the specimen to be polished. Electrical connection to theanode is made through spring which is adapted to hold specimen 24closely if the polishing condition, that is, polishing pI-I, prevails inthe solution, the outer corners and edges of the specimen will veryquickly begin to appear bright. If inspection shows that the specimen isnot being polished but is being etched,

the solution is made slightly more alkaline by the, addition of a fewdrops of strong alkali until observation shows that polishing is takingplace. If inspection of the specimen after a few seconds of preliminaryelectrolysis shows it is being oxidized, the solution is made slightlymore acidic,

It is a relatively" proceed smoothly and a specimen, with little pre- Vliminary mechanical preparation or finishing,

can usually be polished within a matter of 15 to 60 seconds at a currentdensity of about 0.3 to 3.0 amperes per square-centimeter.

The polishing pH for the bath depends upon the temperature of thesolution,the temperature of the anode and the current density as well asupon the anode material. I have found that best polishing conditions areattained by maintaining the anode as cold as possible without causingprecipitation of electrolyte. For example, in carrying out the method,cooling of the anode by contacting it with a freezing mixture or solidcarbon dioxide, either continuously or intermittently, will maintain theanode sufficiently cold to promote the polishing.

Referring to the drawing, Figure 1 illustrates a convenient polishingapparatus in which I!) represents a vessel for electrolyte H. Battery [2represents a source of current, I3 the cathode and I4 the anode which,for convenience in polishing small specimens, can be made from a pieceof metal tubing one end of which has a slit IE to provide a sort ofspring grip for specimen 16. Since it is desired to chill the specimento a temperature well below that of the solution, a freezing mixture 11,conveniently Dry Ice, is placed in the specimen holder.

As has been mentioned, adjustment of the solution to polishing pH isdone empirically by obagainst the bottom of vessel 20. The electrolytemakes contact with the anode through the hole 26 in the bottom of thevessel. The anode can be cooled by placing it in contact with a freezingmixture 21 as shown. By using such an apparatus it is possible to adjustthe solution to polishing pH readily for the anode is at all timesvisible. Also, no interruption in its cooling or electrolysis isnecessary to determine its state. By making the hole in the bottom ofthe vessel a definite area, such as an integral number of squarecentimeters, control of the current density employed is facilitated.

The nature of the electrolytic bath and the method of polishingaccording to the principles of my invention will be more clearlyunderstood by reference to the following specific examples whichcomprise brief descriptions of a few embodiments of my invention.

Example I To 10 milliliters of a saturated aqueous sodium nitratesolution there was added milliliters of ethyl alcohol. The solutioncontaining the sodium nitrate and the alcohol was cooled to about 10 C.and was used to polish a specimen of relatively pure iron afteradjustment to polishing pH of a value slightly more than 7 by theaddition thereto of a few drops of 1.0 normal sodium hydroxide solution.

Cells like those shown in the drawings, consisting of the aboveelectrolytic bath, a, stainless steel cathode and the specimen as theanode were used. The anode was chilled to a temperature of about minus30 C. by placing it in contact with a freezing mixture, whichconveniently can be a cake of Dry Ice, and then dipped into the solutionand subjected to electrolysis at a current density of 0.3 ampere persquare centimeter.

Example II A sample of pearlitic steel was polished according to themethod described at a current density of 0.3 ampere per squarecentimeter in a bath consisting of milliliters of ethyl alcohol and 10milliliters of saturated aqueous sodium nitrate solution, which bath hadbeen slightly alkalized with a few drops of 1.0 normal sodium hydroxide.Under such conditions twenty seconds were required to polish thespecimen.

Example III A sample of Armco iron, more than 99 per cent iron, waspolished according to the method outlined at a current density of 0.3ampere per square centimeter in a bath consisting of 100 milliliters ofethyl alcohol and 15 milliliters of saturated aqueous potassium nitratesolution, which bath has been alkalized with a few drops of 1.0 normalpotassium hydroxide,

Example IV A sample of austenitic steel was polished at a currentdensity of 0.6 ampere per square centimeter in an electrolyte consistingof 10 milliliters of saturated aqueous sodium nitrate solution and 125milliliters of methyl alcohol. The solution was alkalized with a fewdrops'of 1.0 normal soassurance :dium hydroxide and chilled to:0 .C. Theanode :waschilled tosabout1minusr30 C. by contacting;:it "with solidcarbon dioxide .during sthe 'el'ectro'lysis- About forty seconds wererequired forlc'om- .plete polishing.

Example V Samples of 0.45 carbon steel, air-,cooledffrom 1550 F.,werep'olished at 0.43. ampere per square centimeter in difi'erent bathsconsisting of "15 milliliters 'of saturated aqueous-potassium Iiitra'tesolution and 125 milliliters of methyl alcohol, ethyl alcohol andacetone slightly alkalized with a'few drops of 1.0 normalsodium'hydroxide. The solutions were chilled to temperatures vof about0 to l0C. andthespecimens.chilledwith ,solid carbon dioxide during theelectrolysis.

"Similar samples of steel tempered at 1300 F. werepolished in the samebaths under'like temperature conditions at current densities of 0.2ampere persquare centimeter.

Example VI "Asample of stainlesssteel of the'l8 8 variety "Was polishedin an electrolytic 'bath consisting of 15 milliliters of saturatedaqueous sodium ni- "trate solution, 75 milliliters of methyl alcohol and0:1 milliliter of 1.0 normal nitric acid. The-polishing required 30seconds in the aggregate and was accomplished-by chilling the sample .bycon- "tacting it with a freezingmixture giving a tem- .perature of aboutminus 40 C., electroly'zing for about seconds, cleaning the electrolyzedsurface by gentle brushing andrepeating the .steps.

Asimilar sample of"18-'8 stainless steel waspolished in a solutionconsisting of millilitersof saturated aqueous sodium nitrate .solution,"75 milliliters of acetone and"0'.2..m'ill'iliter of 110 :nornial sodiumhydroxide.

A specimen of stainless steel was polished to a, high luster using thetechnique describedand tion was maintained at room temperature while theanode was chilled with a freezing mixture giving the temperature ofabout'minus e40 C.

Example VIII A sample of brass composed of about per cent copper and 30per centrzinc :was polished in I a bath consisting of 5 milliliters ofsaturated aqueous sodium nitrate solution, milliliters of ethyl alcoholand 0.1 milliliter of 1.0 normalnitric .acidata current..densityiofl0.2ampereper square centimeter in about 45seconds. "The anode-was chilledwith a freezing mixture-during the polishing. The solution also polishedwithout the addition of the nitric'acid but it did not give a polish ofas high luster nor did it polish as rapidly as the acidified solution.

The solution was also used in substantially the same manner-.toapolishsamples of phosphor bronze.

When polishing :a binary or "multi cornponent alloy such as"brassprbearing metal; it "is'possi- 1-:ble my slow adjustment of the pHtoward the :ideal polishing value to .find a point at which .one phaseis etched and the other polished. This wasdone with severalbrasses.bronzes and bearing metals.

Example IX EA sampleof aluminum was'polished in a solu- "tionconsistingor 15 milliliters of saturated sodi- "rentdensity of 025 amperep'er'square centimeter.

nitrate, "'15 milliliters of methyl alcohol and 0.1 milliliter ofLo'normal nitric acid at a cur- It was found, also, that the followingsolution polishedaluminum quite satisfactorily: '75 milliliters ofethylCellosolve, 15 milliliters of saturated aqueous potassium 'riitratesolution and "5 "milliliters of water, which solution was made acid toindicator paper using nitric acid. The samples werepolished attemperatures below that .of the solution. .It was found that 1.0 ampereper square centimeter as a current density was i necessary to produce asatisfactory polish in a reasonably short time when the anode was notcooled to a'temperature substantially lower than that of the bath.

The aluminum was also polished with thefollowing solution: '75milliliters of acetone, 15 milliliters of saturated aqueous sodiumnitrate solution, 10milliliters of waterand a few drops of 1.0normalnitricacid to acidify the solution.

The aboveexamples have been given to illus- "trate the steps of'themethod and the general proportions of constituents used 'to make up thebaths. In a similar'manner, as mentioned above, and in like proportions,the alkalimetal salts of the "mineral acids such as'hydrochloric,hydrobromic, hydroiodic, hydrofluoric, phosphoric,

chromicand sulphuric can be used as the electrolytes in'baths forpolishing purposes.

In general, in the polishing of any of the :meta1s, the best: conditionsfor' obtaining the most lustrous polish seem to prevail when thesolutions are adjusted topl-I values such that uniform "gelatinouslayers of .precipitate areobserved on the surfaces being electrolyzed.

From these .dataan'd descriptions illustrating my invention, itsfeatures will' be apparent. 'The method ofpolishing is generallyapplicable to all metals and consisting in employing a high enoughcurrent density .to induce polishing, maintaining the sample beingpolished at alow temperature,

preferably substantially. below that of thepOlishingv solutionandadjusting the pl-I of the bath to .a value which I have denominated thepolishing pH. Thisterm is descriptive and definesthepl-I at whichpolishing occurs which, in general, is close to the neutral in the rangeof 6"to 8 and could also be described as that value of pI-l at "which agelatinous precipitate of metal oxide or hydroxide forms substantiallyuniformly overthe surfacefbeingpolished. The composition of'tlie .bath,aswill be noted from the data given above, isnot specific for the metalbeing polishedas has been true of. prior art electrolytic polishingbaths.

"be'use'd asithe' electrolyte in apolishing solution "fora'metalrequiring a polishing pH of substan- "tiallythe' value to whichsuch salt, bufiers itself.

This class of materials is well "exemplified bywhe borates, somephosphates, acetates and citrates and other organic and inorganic saltsas described in greater detail in my copending. application SerialNumber 600,631, filed of even date herewith, (now Patent Number2,506,582). A second ingredient of the solutions is a completelywatermiscible organic or non-ionizing liquid or solvent which apparentlyfunctions to reduce the ion velocity in the solution to aid in theestablishment of concentration polarization at the anode. The finalingredient of the solutions is merely acid or base to adjust the pH tothe polishing value.

The polished surfaces appear to develop a high degree of resistance tocorrosion for I have found that polished spots on samples remain brightwhen left exposed to the atmosphere, whereas adjacent polished areascorrode rapidly.

All of the metals polished according to the examples given above werefinished to a Very high luster and the surfaces showed no signs ofscratches or defects when reproduced and examined under the electronmicroscope at 50,000X.

Since certain changes in carrying out the electrolytic polishing methodcan be made in the individual steps thereof and wide modifications inthe composition of the electrolytic baths which embody the invention canbe made without departing from the scope of the invention, it isintended that all matter contained in the above discussion, descriptionand examples shall be interpreted as illustrative and not in a limitingsense,

The invention described herein may be manufactured and used by or forthe Government of the United States of America for governmental purposesWithout the payment of any royalties thereon or therefor.

Having described my invention, what I claim as new and desire to secureby Letters Patent of the United States is:

1. The method of electrolytically polishing metals of the groupconsisting of ferrous metals, nickel, brass and aluminum comprisingmaking a specimen of the metal to be polished an anode in anelectrolytic cell in which the electrolyte consists essentially of amixture of a concentrated aqueous solution of an alkali metal salt of anacid selected from the group consisting of nitric, sulfuric, chromic,hydrochloric, hydrobromic, hydroiodic and hydrofluoric acids and acompletely water miscible alcohol, adjusting the pH of the mixture tobetween about 6 and 8, chilling the anode to a temperature below that ofthe electrolyte, and passing an electric current through said cell.

2. The method of electrolytically polishing metals of the groupconsisting of ferrous metals, nickel, brass and aluminum comprisingmaking a specimen of the metal to be polished an anode in anelectrolytic cell in which the electrolyte consists essentially of amixture of a concentrated aqueous solution of an alkali metal salt of anacid selected from the group consisting of nitric,

sulfuric, chromic, hydrochloric, hydrobromic, hy-" droiodic andhydrofluoric acids and a completely water miscible liquid selected fromthe group consisting of methyl alcohol, ethyl alcohol, glycerine,ethylene glycol, acetone and isopropyl alcohol, adjusting the pH of themixture to between about 6 and 8, chilling the anode to a temperature atleast 10 C. below that of the through said cell.

i ,3. The method of electrolytically polishing stainless steelcomprising, making a specimen of said metal the anod in an electrolyticcell in which the electrolyte consists essentially of a mixture of aconcentrated aqueous solution of an alkali metal salt of nitric acid andethyl alcohol, adjusting the pH of the mixture to between about 6 and 8,chilling the anode to a temperature at least 10 C. below that of theelectrolyte, and passing an electric current through said cell. 4. Themethod of electrolytically polishing medium-carbon steel comprisingmaking a specimen of said metal the anode in an electrolytic cell inwhich the electrolyte consists essentially of a mixture of aconcentrated aqueous solution of an alkali metal salt of nitric acid anda mixture of methyl alcohol, ethyl alcohol and acetone, adjusting the pHof the mixture to between about 6 and 8, chilling the anode to atemperature at least 10 C. below that of the electrolyte, and passing anelectric current through said cell. 5. The method of electrolyticallypolishing stainless steel comprising making a specimen of said metal theanode in an electrolytic cell in which the electrolyte consistsessentially of a mixture of a concentrated aqueous solution of an alkalimetal salt of nitric acid and glycerine, adjusting the pH of the mixtureto between about 6 and 8, chilling the anode to a temperature at least10 C. below that of the electrolyte, and passing an electric currentthrough said cell.

6. The method of electrolytically polishing aluminum comprising making aspecimen of said metal the anode in an electrolytic cell in which theelectrolyte consists essentially of a mixture of a concentrated aqueoussolution of an alkali metal salt of nitric acid and methyl alcohol,adjusting the pH of the mixture to between about 6 and 8, chilling theanode to a temperature at least 10 C. below that of the electrolyte, andpassing an electric current through said cell.

'7. The method of electrolytically polishing brass comprising making aspecimen of said metal the anode in an electrolytic cell in'which thelectrolyte consists essentially of a mixture of a concentrated aqueoussolution of an alkali metal salt of nitric acid and ethyl alcohol,adjusting the pH of the mixture to between about 6 and 8, chilling theanode to a temperature at least 10 C. below that of the electrolyte, andpassing an electric current through said cell.

EDWARD DER MATEOSIAN.

REFERENCES CITED The following references are of record in the file ofthis patent:

UNITED STATES PATENTS OTHER REFERENCES Proceedings of The AmericanElectroplaters SocietyXf June 1946, pages and 64.

1. THE METHOD OF ELECTROLYTICALLY POLISHING METALS OF THE GROUPCONSISTING OF FERROUS METALS, NICKEL, BRASS AND ALUMINUM COMPRISINGMAKING A SPECIMEN OF THE METAL TO BE POLISHED AN ANODE IN ANELECTROLYTIC CELL IN WHICH THE ELECTROLYTE CONSISTS ESSENTIALLY OF AMIXTURE OF A CONCENTRATED AQUEOUS SOLUTION OF AN ALKALI METAL SALT OF ANACID SELECTED FROM THE GROUP CONSISTING OF NITRIC, SULFURIC, CHROMIC,HYDROCHLORIC, HYDROBROMIC, HYDROIODIC AND HYDROFLUORIC ACIDS AND ACOMPLETELY WATER MISCIBLE ALCOHOL, ADJUSTING THE PH OF THE MIXTURE TOBETWEEN ABOUT 6 AND 8, CHILLING THE ANODE TO A TEMPERATURE BELOW THAT OFTHE ELECTROLYTE, AND PASSING AN ELECTRIC CURRENT THROUGH SAID CELL.